This invention relates to soldering and fluxes used in soldering processes. More particularly, this invention relates to soldering fluxes useful in high-temperature soldering processes.
Soldering is one of the oldest methods of joining two or more metal articles. Soldering involves the joining of metallic surfaces by flowing between them by capillary attraction alloys of lower melting point than the metals to be joined. The metal surfaces remain unmelted, but are solidly united when the solder alloy or solder metal solidifies.
Preparation of the joint surfaces is an important factor in soldering. With few exceptions, a flux composition is utilized. Fluxes are used to prevent the oxidation of the filler metal and of the surfaces of the metal being joined during the heating. The flux will also dissolve oxides which naturally exist on most surfaces as well as those that may form during the heating operation. Additionally, fluxes influence the surface-tension equilibrium in the direction of solder spreading.
Fluxes are applied to the metal surfaces to be soldered by many methods, for example brushing, spraying, dipping, etc. However, utilization of a liquid vehicle for application of the flux has many disadvantages in many processes. For example, in the assembly of automotive radiators, the parts to be soldered are commonly dipped in a flux-containing bath. Dipping of the part has many disadvantages. First, it results in a large waste of material, since the entire part is coated with fluxing material rather than just the surface to be soldered; secondly, the solvent must be evaporated first before the soldering occurs, and this results in a loss of energy; thirdly, use of a solvent-or water-based flux creates air pollution problems due to emissions of volatile constituents into the atmosphere.
The aforementioned problems can be overcome through utilization of a wax flux. Wax-flux compositions are known in the art and have been used in low-temperature soldering processes. See, for example, U.S. Pat. Nos. 3,977,916; 3,960,614; 3,960,613; 3,975,216; and 3,954,494, which discloses wax-flux compositions containing sulfonic acids. Wax-flux compositions avoid some of the prior art problems encountered with liquid fluxes in that the flux can be readily applied only to the surface to be soldered, thus avoiding waste of flux. Furthermore, the wax quickly solidifies so that the parts can be easily stored for future soldering. However, wax-flux compositions have not been utilized in higher-temperature soldering for many reasons, but particularly because of the auto-ignition hazards caused by the wax component of the flux.
The use of inorganic gases and liquid organic halogen-containing fluxes is known in the art. See, for example, pages 19-20 of Manko, Solders and Soldering, McGraw-Hill (1964). One problem with the organic halogen-containing fluxes is that of corrosion. The organic halogen-containing fluxes are extremely temperature-sensitive by virtue of the organic radical and the halogen and acids formed therefrom are extremely corrosive. Also, because of their hazardous nature, special equipment is required for safety reasons. Furthermore, any residual flux or flux residues must be removed from the part soldered to prevent future corrosion.